Row driver for selectively supplying operating power to imager pixel

ABSTRACT

An imaging system includes a pixel that does not require a row select transistor. Instead, an operating voltage is selectively provided to the pixel&#39;s readout circuitry, and the readout circuitry provides output signals based on charge or voltage of a storage node. The operating voltage can be selectively provided to each row of a pixel array by a row driver. Each pixel includes a source follower transistor that provides an output signal on a column output line for readout. An anti-blooming transistor may be linked to each pixel&#39;s photosensor to provide an overflow path for electrons during charge integration, prior to transfer of charge to the pixel&#39;s storage node by a transfer transistor. Electrons not produced by an image are introduced to the photosensor prior to image acquisition, filling traps in the photosensor to reduce image degradation.

This application is a divisional of U.S. patent application Ser. No.11/265,154, filed on Nov. 3, 2005 now U.S. Pat No. 7,176,434, which is adivisional of U.S. patent application Ser. No. 10/766,012 filed on Jan.29, 2004 now U.S. Pat No. 7,196,304, both of which are incorporatedherein, by entirety.

FIELD OF THE INVENTION

The invention relates generally to improving the control and operationof an imager pixel.

BACKGROUND OF THE INVENTION

An imager, for example, a CMOS imager includes a focal plane array ofpixel cells; each cell includes a photosensor, for example, a photogate,photoconductor or a photodiode overlying a substrate for producing aphoto-generated charge in a doped region of the substrate. A readoutcircuit is provided for each pixel cell and typically includes at leasta source follower transistor and a row select transistor for couplingthe source follower transistor to a column output line. The pixel cellalso typically has a charge storage node, for example, a floatingdiffusion node which is, in turn, connected to the gate of the sourcefollower transistor. Charge generated by the photosensor is stored atthe storage node. In some arrangements, the imager may also include atransistor for transferring charge from the photosensor to the storagenode. The imager also typically includes a transistor to reset thestorage node before it receives photo-generated charges.

In a CMOS imager pixel cell, for example, a four transistor (4T) pixelcell 100 as depicted in FIG. 1, the active elements of a pixel cellperform the functions of (1) photon to charge conversion by photodiode102; (2) transfer of charge to the floating diffusion node 108 by thetransfer transistor 104; (3) resetting the floating diffusion node to aknown state before the transfer of charge to it by reset transistor 106;(4) selection of a pixel cell for readout by row select transistor 112;and (5) output and amplification of a signal representing a resetvoltage and a pixel signal voltage based on the photo converted chargesby source follower transistor 110, which has its gate connected to thefloating diffusion node 108. The pixel of FIG. 1 is formed on asemiconductor substrate as part of an imager device pixel array.

FIG. 2 illustrates a block diagram of a CMOS imager device 908 having apixel array 200 with each pixel cell being constructed as describedabove, or as other known pixel cell circuits. Pixel array 200 comprisesa plurality of pixels arranged in a predetermined number of columns androws (not shown). The pixels of each row in array 200 are all turned onat the same time by a row selected line, and the pixels of each columnare selectively output by respective column select lines. A plurality ofrows and column lines are provided for the entire array 200. The rowlines are selectively activated in sequence by the row driver 210 inresponse to row address decoder 220 and the column select lines areselectively activated in sequence for each row activation by the columndriver 260 in response to column address decoder 270. Thus, a row andcolumn address is provided for each pixel.

The CMOS imager is operated by control circuit 250, which controlsaddress decoders 220, 270 for selecting the appropriate row and columnlines for pixel readout, and row and column driver circuitry 210, 260which apply driving voltage to the drive transistors of the selected rowand column lines. The pixel output signals typically include a pixelreset signal V_(rst) taken off of the floating diffusion node 108 whenit is reset by reset transistor 106 and a pixel image signal V_(sig),which is taken off the floating diffusion node 108 after photo-generatedcharges generated by an image are transferred to it. The V_(rst) andV_(sig) signals are read by a sample and hold circuit 265 and aresubtracted by a differential amplifier 267, which produces a signalV_(rst)-V_(sig) for each pixel, which represents the amount of lightimpinging on the pixels. This difference signal is digitized by ananalog to digital converter 275. The digitized pixel signals are thenfed to an image processor 280 to form a digital image. The digitizingand image processing can be located on or off the imager chip. In somearrangements the differential signal V_(rst)-V_(sig) can be amplified asa differential signal and directly digitized by a differential analog todigital converter.

As shown in FIG. 1, the conventional four transistor (4T) pixel requiresan operating voltage Vcc, as well as transfer TG, row select ROW andreset RST control signals for operation.

BRIEF SUMMARY OF THE INVENTION

Method and apparatus embodiments of the present invention provide a newpixel design for an imager in which the pixel is operated with a rowdriver signal which supplies operating power and selects the pixel foroperation and readout, and a reset and transfer control signal.

In one exemplary embodiment, the pixel cell includes a photosensor, atransfer transistor operated by a transfer control signal, a storagenode for receiving transferred charges from the photosensor, resettransistor for the resetting of the storage node operated by a resetcontrol signal and output transistor having a gate coupled to thestorage node and receiving operating power and providing a selectivereadout in response to the row driver signal.

In another aspect of the exemplary embodiment, the pixel cell having theforegoing construction can be operated to precharge the photosensor withelectrons to mitigate against loss of photo-generated image chargesduring charge transfer to the storage node.

In another exemplary embodiment, the pixel cell includes ananti-blooming transistor coupled to the row driver signal andphotosensor which provides an overflow path for electrons to reduce oversaturation of the photosensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages and features of the invention willbecome more apparent from the detailed description of exemplaryembodiments provided below with reference to the accompanying drawingsin which:

FIG. 1 is a schematic diagram of a conventional four transistor pixel;

FIG. 2 is a block diagram of a conventional imager device;

FIG. 3 is a schematic circuit diagram according to a first embodiment ofthe invention;

FIG. 4 is a timing diagram for shutter operation of the FIG. 3embodiment;

FIG. 5 is a voltage threshold diagram for a pinned photodiode used inthe FIG. 3 embodiment;

FIG. 6 is a timing diagram for charge readout of the FIG. 3 embodiment;

FIG. 7 is a schematic circuit diagram of a pixel according to a secondembodiment of the invention;

FIG. 8 is a timing diagram for shutter operation of the FIG. 7embodiment; and

FIG. 9 is a diagram of a processing system which employs an imageremploying an array of pixels constructed in accordance with the variousembodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings, which are a part of the specification, and inwhich is shown by way of illustration various embodiments whereby theinvention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to make and use theinvention. It is to be understood that other embodiments may beutilized, and that structural, logical, and electrical changes, as wellas changes in the materials used, may be made without departing from thespirit and scope of the present invention. Additionally, certainprocessing steps are described and a particular order of processingsteps is disclosed; however, the sequence of steps is not limited tothat set forth herein and may be changed as is known in the art, withthe exception of steps or acts necessarily occurring in a certain order.

The terms “wafer” and “substrate” are to be understood asinterchangeable and as including silicon, silicon-on-insulator (SOI) orsilicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxiallayers of silicon supported by a base semiconductor foundation, andother semiconductor structures. Furthermore, when reference is made to a“wafer” or “substrate” in the following description, previous processsteps may have been utilized to form regions, junctions or materiallayers in or on the base semiconductor structure or foundation. Inaddition, the semiconductor need not be silicon-based, but could bebased on silicon-germanium, germanium, gallium arsenide, or other knownsemiconductor materials.

The term “pixel” refers to a photo-element unit cell containing aphotoconversion device or photosensor and transistors for processing anelectrical signal from electromagnetic radiation sensed by thephotoconversion device such as imager 908 (FIG. 2). The embodiments ofpixels discussed herein are illustrated and described as employing threetransistor (3T) or four transistor (4T) pixel circuits for the sake ofexample only. It should be understood that the invention may be usedwith other pixel arrangements having more than three transistors.

Although the invention is described herein with reference to thearchitecture and fabrication of one pixel cell, it should be understoodthat this is representative of a plurality of pixels in an array of animager device such as imager 908 (FIG. 2). In addition, although theinvention is described below with reference to a CMOS imager, theinvention has applicability to any solid state imaging device havingpixels. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present invention isdefined only by the appended claims.

FIG. 3 illustrates an exemplary circuit 300 for a pixel of a CMOS imageraccording to a first exemplary embodiment of the invention. The pixelincludes a photosensor, e.g. a photodiode 302, transfer transistor 304,a floating diffusion node 306, and a reset and readout circuit includingreset transistor 308, source follower transistor 312 and row drivercircuit 310. Row driver circuit 310 supplies operating power and resetvoltage levels to the pixel 300 in the form of a row driver signalROW_DRIVER, which can be provided in the same manner as row driver 210(FIG. 2). It should be understood, that when the pixel 300 is employedin a pixel array, a plurality of like pixels are arranged in rows andcolumns and all pixels in a row receive ROW_DRIVER from a common rowdriver circuit 310 while all pixels in a column are coupled to provideoutput signals on a common column line 313.

The illustrated pixel 300 is formed on a semiconductor substrate. Thephotodiode 302 generates and accumulates signal charge in response toincident light during a charge integration period. After the integrationperiod, the charge is transferred via the transfer transistor 304 to thefloating diffusion node 306. Shutter timing signals are used to initiatea charge integration operation of the pixel 300.

FIG. 4 illustrates the shutter timing signals for operating the FIG. 3pixel. Referring now to FIGS. 3 and 4, initially, at time t1, the signalROW_DRIVER is set low and thereafter the gate control signals fortransfer transistor 304 (TX_EN) and reset transistor 308 (RST_EN) areset high at time t2 causing electrons to fill photodiode 302 andfloating diffusion node 306. ROW_DRIVER is provided to a channelterminal of reset transistor 308, illustratively the source terminal.The electrons are drawn to the photodiode 302 and floating diffusionnode 306 because both photodiode 302 and floating diffusion node 306 areinitially at a higher potential Vpin, for example, 2.3 volts. WhenROW_DRIVER is set low the actual voltage of ROW_DRIVER still remainsabove ground (for example, 0.1 volts) to prevent an injection ofelectrons into the substrate. With row driver 310 set low and an influxof electrons flowing into photodiode 302 and floating diffusion node306, the potential of photodiode 302 and floating diffusion node 306 issubsequently reduced to, for example, 0.1 volts.

Next, while the gate control signals for transfer transistor 304 andreset transistor 308 remain high, the ROW_DRIVER signal is set high attime t3. This drains the electrons from photodiode 302 and floatingdiffusion node 306, thereby resetting photodiode 302 to Vpin. Inaddition, any electron traps within photodiode 302 caused by an inherentbarrier voltage Vbarrier (FIG. 5 discussed below) are filled withelectrons previously introduced to photodiode 302 when ROW_DRIVER wasset low. The timing illustrated at t2 and t3 is for precharging thephotosensor with electrons to mitigate against the loss ofphoto-generated image charges during charge transfer to the storagenode, this precharging is an optional operation.

Next, while RST_EN and ROW_DRIVER remain high, TX_EN is set low at timet4. This turns off transfer transistor 304 and photodiode 302 is nowisolated from the floating diffusion node 306. With photodiode 302 resetto Vpin and isolated from floating diffusion node 306, chargeintegration for photodiode 302 now begins.

Next, while RST_EN is high and TX_EN is low, ROW_DRIVER is set low attime t5. Setting ROW_DRIVER low forces the floating diffusion node 306to a potential of approximately 0.1 volts, which turns off sourcefollower transistor 312 to isolate the output bus shared by pixels ineach column.

Next, while TX_EN and ROW_DRIVER are low, RST_EN is set also low at timet6. This turns off reset transistor 308 low and floating diffusion node306 is isolated from the ROW_DRIVER signal. Since the floating diffusionnode 306 is also now set low there is no output from the source followertransistor 312.

The shutter timing signals in FIG. 4 can be applied concurrently to allpixels in an imager array 200 (FIG. 2). As a result, all photodiodes 302in the array begin integration concurrently and all source followertransistors 312 are turned off concurrently by setting all floatingdiffusion nodes 306 to a turn off voltage, for example 0.1 volts.

FIG. 5 illustrates the potential diagram of the photodiode 302, transfergate TG and floating diffusion node FD. When the transfer transistor 304is turned off by TX_EN going low at time t4, a barrier exists impedingcharge transfer between photodiode 302 and floating diffusion node 306which is shown by A. While the barrier A exists between the photodiode302 and floating diffusion node 306, charge integration from a signalproduced by light incident on the photodiode 302 occurs. Subsequently,as described below, when TX_EN goes high again, the barrier A falls tothe level of Vbarrier allowing electrons to transfer from the photodiode302 to the floating diffusion node 306. However, even when the transfergate 304 is on, a small barrier may still remain so that not allelectrons in the photodiode 302 are transferred to the floatingdiffusion node 306. Some electrons remain trapped at photodiode 302.However, because the photodiode 302 was initially coupled to theROW_DRIVER signal, the trap was filled, i.e., precharged, with non-imageelectrons, so image electrons acquired in photodiode 302 duringintegration are not lost during the charge transfer to floatingdiffusion node 306 due to the small remaining barrier between thephotodiode 302 and floating diffusion node 306 when the transfertransistor 304 is on.

FIG. 6 illustrates a timing diagram for the FIG. 3 circuit 300 duringpixel readout, which occurs after the shutter timing depicted in FIG. 4and the charge integration at photodiode 302 is completed. Initially,the floating diffusion nodes 306 of all pixels in an imager array 200(FIG. 2) are set to a predetermined voltage, for example 0.1 volts toensure that all source follower transistors 312 are turned off asdescribed above when ROW_DRIVER is low, TX_EN is low and RST_EN is highat time t5 (FIG. 4). The ROW_DRIVER signal of the pixel intended to beread is pulsed high at time t8 (FIG. 6) providing an operating voltageacross source follower transistor 312. The floating diffusion node 306of the pixel intended to be sampled is then reset at time t9 by brieflyturning on reset transistor 314 supplied with operating voltage byROW_DRIVER and signal RST_EN going high, thereby resetting floatingdiffusion node 306 to a predetermined ROW_DRIVER voltage (for example,3.3 volts). The reset voltage level on the floating diffusion node 306is then applied to the gate of source follower transistor 312, whichconverts it to a reset output voltage V_(rst) on a column output line313 (FIG. 3). The output signal is subsequently sampled at time t10, forexample by a sample and hold circuit 265 (FIG. 2), where a high pulseSHR is used to sample and hold the reset output voltage V_(rst) onto afirst sample and hold capacitor.

Charge stored in photodiode 302 from a previous integration period isthen transferred to floating diffusion node 306 by signal TX_EN goinghigh at time t11 thereby, turning on transfer transistor 304 andlowering the potential barrier in FIG. 5 to Vbarrier. The transferredcharge lowers the voltage on the floating diffusion node 306 to a pixeloutput signal level, which is applied to the gate of source followertransistor 312. Source follower transistor 312, which is still suppliedwith operating voltage by ROW_DRIVER being high, converts the signalvoltage level to a signal output voltage V_(sig) on the column outputline. Sample and hold circuit 265 (FIG. 2) in response to a sample/holdpulse SHS at time t12 causes the pixel's signal output voltage V_(sig)on the column line to be stored in a second sample and hold capacitor.

After V_(sig) is sampled, the ROW_DRIVER signal is set to a low voltageabove ground, for example 0.1 volts at time t13. Reset transistor 314 isbriefly turned on again at time t14, setting floating diffusion node 306to a low voltage, for example 0.1 volts and the pixel circuit is readyfor a next image capture frame.

Since transfer transistor 304 is positioned between photodiode 302 andfloating diffusion node 306, the floating diffusion node 306 can bereset prior to transferring electrons. This permits a correlated doublesampling operation resulting in reduced kTC noise and image noise. Aglobal array signal may be implemented to control multiple row drivers310 and the reset transistors 308 in an imager array; however, thetransfer transistor 304 and the reset transistor 308 are controlledindividually for each pixel.

As noted above with respect to the FIG. 5 potential diagram, when thetransfer transistor 304 is on at time t11, all of the charge stored inthe photodiode 302 may not be transferred to the floating diffusion node306 because the pinning potential Vpin of the photodiode 302 is higherthan the barrier voltage Vbarrier. As a result, some of the electronsgenerated during the integration period are trapped at the photodiode302. However because the trap sites were initially filled at time t2with non-image electrons, no image charges are lost.

FIG. 7 illustrates a modified pixel circuit 400 according to a secondexemplary embodiment of the invention. This modified embodiment adds tothe FIG. 3 pixel 300 an anti-blooming transistor 414 coupled to voltagesource VAA, which is a high voltage, for example 3.3 volts. Theanti-blooming transistor 414 is controlled by gate signal AB.

Pixel circuit 400 operates like pixel circuit 300; however, pixelcircuit 400 includes an anti-blooming transistor 414 to provide anoverflow path when photodiode 302 is approaching saturation, to drainexcess charge from photodiode 302 when it is overexposed during chargeintegration. It should be noted that even when transistor 414 is turnedon by control signal AB, a high charge barrier VAB (FIG. 5) is presentbetween the photodiode 302 and anti-blooming transistor 414 that must beovercome, i.e. when photodiode 302 approaches saturation, beforeoverflow charges are drained and flow through transistor 414.

FIG. 8 illustrates the shutter timing diagram for the FIG. 7 circuit.The control signals ROW_DRIVER, TX_EN and RST_EN operate according tothat of pixel circuit 300 (FIG. 4). In addition, control signal ABcontrols the operation of anti-blooming transistor 414. When ROW-DRIVERand RST_EN are set high and TX_EN transitions from high to low,beginning charge integration in photodiode 302, signal AB is set to aconstant voltage, for example 0.1 volts. This allows anti-bloomingtransistor 414 to provide a blooming path out of pixel circuit 400. Withanti-blooming transistor 414 on during charge integration, the overflowpath is created from the photodiode 302 to reduce over saturation byphotodiode 302 during image acquisition, i.e., charge integration.Charge readout occurs using the same signal timing as that of the FIG. 3circuit.

In a variation to the shutter timing for the FIG. 7 circuit, the ABsignal could be pulsed high then held at a constant voltage, for example0.1 volts, prior to charge integration. The pulsing of signal AB resultsin resetting pixel circuit 400. When signal AB is held at a constantvoltage, an anti-blooming path is provided out of pixel circuit 400. Inthis instant, signal AB is pulsed and held for the rows that will beused to acquire an image, i.e., rows performing charge integration.

If the floating diffusion node 406 becomes saturated during chargetransfer from photodiode 402 and electrons begin to overflow, theseelectrons will overflow into row driver 410 thereby providing ananti-blooming path for floating diffusion node 406.

FIG. 9 shows a processor system 900, which includes an imaging device908 which is the same as FIG. 2, but as modified to use the pixelsdescribed herein. The imager device 908 may receive control or otherdata from system 900. System 900 includes a processor 902 having acentral processing unit (CPU) that communicates with various devicesover a bus 904. Some of the devices connected to the bus 904 providecommunication into and out of the system 900; an input/output (I/O)device 906 and imager device 908 are such communication devices. Otherdevices which may be connected to the bus 904 provide memory,illustratively including a random access memory (RAM) 910, hard drive912, and one or more peripheral memory devices such as a floppy disk orother memory drive 914 and compact disk (CD) drive 916. The imagerdevice 908 may, in turn, be coupled to processor 902 for imageprocessing, or other image handling operations. Examples of processorsystems, which may employ the imager device 908, include, withoutlimitation, computer systems, camera systems, scanners, machine visionsystems, vehicle navigation systems, video telephones, surveillancesystems, auto focus systems, image stabilization systems, and others.

The devices described above illustrate typical devices of many thatcould be used. The above description and drawings illustrate anembodiment, which achieves the features and advantages of the presentinvention. However, it is not intended that the present invention bestrictly limited to the above-described and illustrated embodiment. Anymodifications, though presently unforeseeable, of the present inventionthat come within the spirit and scope of the following claims should beconsidered part of the present invention.

1. A method of operating pixels of an image sensor comprising: applyinga first voltage to a photosensor to flood the photosensor withpre-charge electrons; applying a second voltage to the photosensor todrain a portion of the pre-charge electrons off the photosensor, wherethe second voltage is higher than the first voltage; accumulatingphoto-generated charge in the photosensor from an image to be capturedby the image sensor; providing an operating voltage across a sourcefollower transistor; setting a floating diffusion node, which is coupledto a gate of the source follower transistor, to a high reset voltagethat exceeds a threshold turn-on voltage of the source followertransistor; turning on a transfer transistor to transfer a charge fromthe photosensor to the floating diffusion node, changing the charge atthe floating diffusion node to a signal voltage; and reading a signalprovided by the source follower transistor in response to the signalvoltage.
 2. The method of claim 1, further comprising setting thefloating diffusion node to a reset voltage lower than the thresholdturn-on voltage of the source follower transistor to turn off the sourcefollower transistor.
 3. The method of claim 1 further comprising turningon an anti-blooming transistor to transfer overflow charge from aphotosensor.
 4. A method of operating a pixel, the method comprising:applying a first operating voltage to a photosensor in the pixel tostore non-image electrons in the photosensor prior to accumulatingphoto-generated charge from an image to be captured by the pixel;applying a second operating voltage to the photosensor to drain at leastsome of the non-image electrons from the photosensor; initiating anintegration period of accumulating photo-generated charge in thephotosensor from light from the image; and providing the first andsecond operating voltages by a row driver; wherein the first operatingvoltage is applied to the photosensor through the following steps:lowering the row driver voltage to the first voltage, where the firstvoltage is lower than the second voltage; turning on a reset transistorto connect the row driver voltage to a floating diffusion node in thepixel; and turning on a transfer transistor to connect the floatingdiffusion node to the photosensor.
 5. The method of claim 4, wherein thefirst operating voltage is lower than the second operating voltage. 6.The method of claim 4, wherein the step of applying the second voltageto drain non-image electrons comprises the following steps: maintaininga signal to the reset transistor sufficient to keep the reset transistoron; maintaining a signal to the transfer transistor sufficient to keepthe transfer transistor on; and raising the row driver voltage to thesecond voltage.
 7. The method of claim 6, wherein the first and secondvoltages are applied globally to the pixel array.
 8. The method of claim4, further comprising deselecting a pixel for readout through thefollowing steps: lowering the row driver voltage to the first voltage,where the first voltage is lower than the second voltage; and turning onthe reset transistor to connect the row driver voltage to the floatingdiffusion node, thereby turning off a source follower transistor in thepixel.
 9. The method of claim 4, further comprising selecting a pixelfor readout through the following steps: raising the row driver voltageto the second voltage, where the second voltage is higher than the firstvoltage; and pulsing a signal to a gate of a reset transistor in thepixel to connect the row driver to a floating diffusion node in thepixel, thereby turning on a source follower transistor having a gateconnected to the floating diffusion node.
 10. The method of claim 9,further comprising reading out the pixel through the following steps:dropping the signal pulsed to the reset transistor to turn off the resettransistor; pulsing a sample-and-hold-reset signal to sample a signalproduced from the reset charge value at the floating diffusion node;pulsing a signal to a gate of a transfer transistor to transfer chargestored in the photosensor to the floating diffusion node; and pulsing asample-and-hold-signal signal to sample a signal produced from thetransferred charge value at the floating diffusion node.
 11. The methodof claim 10, further comprising deselecting the pixel for readoutthrough the following steps: lowering the row driver voltage to thefirst voltage; and pulsing a signal to the gate of the reset transistorto connect the row driver to the floating diffusion node, therebyturning off a source follower transistor having a gate connected to thefloating diffusion node.